William,
I found the Vanguard 33 on SailboatData.com. She surely has some fine lines. The simplest place to install the mainmast is just in front of the trunk cabin. Here the deck is strong and all you need is a 2 x 2’ plywood backing plate where you are bolting on the (steel) mast partners.
I have one little concern: The integral keel-rudder is fairly short. The rudder, having been tucked quite far forward, may thus have only moderate steering efficiency. I fear that when reaching with a big, broad sloop JR, she may soon run out of rudder. Now, building a second spade-rudder, further aft, would correct this, but would be quite a job. An alternative is to rig her as a JR yawl, that is, with a little mizzen way aft. This mizzen needs only be small, so a triangular sail with a sprit boom would be fine. The biggest advantage with the yawl is that one may offload the rudder by just easing the sheet of the mizzen. A second bonus is that the boat will sit beautifully at anchor with the mizzen set as a riding sail.
I have written some thoughts on this in Chapter 3 (and 4) of “The Cambered Panel Junk Rig”, found here. Maybe you find some of that useful.
Anyway, good luck!
Arne